1. Field of the Invention
The present invention generally relates to noise control, and more particularly, to methods and apparatuses for controlling noise and vibrations generated by rotating blades of a fan.
2. Background of Invention
Virtually all large modern transport aircraft, especially civil aircraft, use turbofan engines. A turbofan engine essentially comprises a turbojet with a large low-pressure compressor, in which airflow is discharged as a relatively cool and slow propulsive jet. The turbofan exhibits superior propulsive efficiency and fuel consumption characteristics over ordinary turbojets. Perhaps the most important advantage of turbofan engines over turbojets in civil aviation, however, is the dramatically lower noise level.
Despite the improvement over conventional turbojets, however, turbofan engines still produce a powerful roar. The sound of turbofan engines continues to generate animosity among the public towards the aviation industry. Noise concerns produce resistance to the construction of new, more convenient airports, and promote strict noise control regulations. Compliance with these regulations often presents an enormous expense to aircraft owners because engines or even entire aircraft may require replacement.
In some cases, compliance may even be impossible. Current solutions for reducing engine noise have met with limited success. Although a turbofan produces less jet noise than a turbojet, the fan itself has become a source of considerable noise. Much of the fan noise may be produced by interaction between unsteady fluid flows and solid surfaces. Such interactions may be attributed to inflow disturbances, such as the inlet boundary layer interacting with the blades of the fan, or the fan wakes and blade tip vortices interacting with the stator vanes or support struts downstream from the fan. In addition, as fans become larger and more powerful, the contribution of the fan to the overall noise of the engine becomes more pronounced.
A typical acoustic spectrum for a fan includes both broadband noise and higher amplitude noise at particular frequencies relating to the fan rotation rate. The higher amplitude noise associated with the fan rotation rate, referred to here as blade-passing noise, is generated, in considerable part, by aerodynamic interaction between blade load or blade thickness disturbance flows and solid surfaces. This interaction occurs, for example, when the downstream stator vanes or support struts respond to flow fluctuations caused by the upstream fan blades. Under high loading conditions, the most significant of such disturbances to the flow downstream are caused by the development of blade leading edge vortices and stronger blade tip vortex shedding. Consequently, fan blade-passing noise is typically most severe under high load conditions.
Broadband noise, on the other hand, is generated from multiple sources. Some broadband noise sources depend on three-dimensional unsteady flows within a rotor or stator blade row. Such three-dimensional flows may comprise a vortex at the blade tip adjacent to the surface, due to the leakage of fluid through the tip clearance gap. Also, three-dimensional flow may include blade tip vortices which are generated by the deflection of the flow streamlines within the annular wall boundary layer. Such leakage and blade tip vortex flows may increase the fan broadband noise.
To minimize fan noise, a noise control system should attenuate both the blade-passing noise as well as the fan broadband noise. Absorbing the fan noise with acoustic wall treatments or engine hushkitters is somewhat effective, but only produces relatively small reductions in fan noise and often adversely affects engine performance. Active noise control solutions, on the other hand, show promise, but the techniques are not fully developed, require precise and costly control systems, and typically address only blade-passing noise corresponding to the rotation rate of the fan.
Furthermore, fan noise presents a problem in many applications other than aircraft engines. For example, cooling fans are very common as household appliances, in computers, and in other applications. In many of these applications, controlling the noise generated by the fan is a common design objective. As fans become more powerful, however, noise control often becomes more difficult.